The catalytic conversion of carbohydrates from a sustainable resource to valuable chemicals such as alkylene glycols has gained interest. Alkylene glycols are interesting chemicals that find application in the preparation of polyesters, such as poly(alkylene terephthalate), poly(alkylene naphthenate) or poly(alkylene furandicarboxylate). Further applications of alkylene glycols, in particular ethylene glycol, include its use as anti-freeze. By enabling the preparation of such chemicals from sustainable resources, the dependence of fossil fuel resources is reduced. Since there is a desire to reduce the dependence of fossil fuels there is a growing need for different sustainable resources for the production of alkylene glycols such as ethylene glycol.
In U.S. Pat. No. 7,960,594 a process is described wherein ethylene glycol is produced from cellulose. This process involves catalytic degradation and hydrogenation reactions under hydrothermal conditions. More in particular, the process is carried out by contacting cellulose at elevated temperature and pressure with a catalyst system comprising two sorts of active components in the presence of hydrogen. The first active component comprises tungsten or molybdenum in its metallic state or its carbide, nitride or phosphide. The second component is selected from the hydrogenation metals from Groups 8, 9 and 10 of the Periodic Table of Elements, and includes cobalt, nickel, ruthenium, rhodium, palladium, iridium and platinum. In experiments the compounds were used on a carrier, such as activated carbon. Moreover, it appears that the reaction conditions that results in satisfactory yields include a temperature of 220-250° C. and a hydrogen pressure of 3 to 7 MPa (measured at room temperature). When a 1% wt slurry of cellulose is subjected to these compounds for 30 minutes, ethylene glycol is obtained in yields of up to 69%. However, it also appears that when the reaction is continued for a prolonged period the ethylene glycol yield reduces.
In U.S. Pat. No. 8,410,319 a continuous process is described wherein a cellulose-containing feedstock is contacted with water, hydrogen and a catalyst to generate at least one alkylene glycol. The catalyst comprises a first metal component selected from the group consisting of Mo, W, V, Ni, Co, Fe, Ta, Nb, Ti, Cr, Zr and combinations thereof. The first metal component is in the elemental state or the metal is the carbide, nitride or phosphide compound. The catalyst further comprises Pt, Pd, Ru and combinations thereof, wherein the metal is in the elemental state. The catalyst components are comprised on a carrier.
This reaction has been further studied on catalyst systems that contain nickel and tungsten on a carrier. There it has been found that nickel and tungsten are leached into the solution during the reaction, which accounts for the gradual deterioration of the catalyst performance (cf. Na Ji et al., ChemSusChem, 2012, 5, 939-944). The leaching of tungsten and other metals has been confirmed in the study reported in M. Zheng et al., Chin. J. Catal., 35 (2014) 602-613. The latter document also discloses that in addition to ethylene glycol different by-products are obtained, including 1,2-propylene glycol, erythritol, glycerol, mannitol and sorbitol.
US 2011/0312488 describes a catalyst system for the generation of alkylene glycols from a carbohydrate as a potential alternative for a catalyst containing the metal components in the elemental state; this catalyst system comprises at least one metal with an oxidation state of at least +2. More in particular this US application discloses a catalyst system comprising a first metal component with an oxidation state of at least +2 and a hydrogenation component. The hydrogenation component can be selected from a wide range of metals in any oxidation state, including in the elemental state. The hydrogenation component may in particular comprise an active metal component selected from the group consisting of Pt, Pd, Ru, Rh, Ni, Ir and combinations thereof. The first metal component may also be selected from a range of metals, but in particular the compounds comprising the first metal component may be selected from the group comprising tungstic acid, molybdic acid, ammonium tungstate, ammonium metatungstate, ammonium paratungstate, tungstate compounds comprising at least one Group 1 or 2 element, metatungstate compounds comprising at least one Group 1 or 2 element, paratungstate compounds comprising at least one Group 1 or 2 element, tungsten oxides, heteropoly compounds of tungsten and various salts and oxides of molybdenum, niobium, vanadium, zirconium, titanium and chromium. The catalyst system according to US 2011/0312488 is stated to improve the selectivity to ethylene glycol and propylene glycol, with a reduced production of butane diols. The ethylene glycol generation is shown in some experiments, indicating that ammonium metatungstate is the preferred first metal component and that as preferred hydrogenation component platinum and nickel may be used. The use of nickel-containing catalyst systems results in the highest yields of ethylene glycol and optionally propylene glycol.
In the above-mentioned article of M. Zheng et al., Chin. J. Catal., 35 (2014) 602-613, the conclusion is drawn that tungsten acid-based catalysts are the most promising candidates for future commercialization of the cellulose-to-ethylene-glycol process. A hydrogenation component is added to such tungsten acid-based catalysts. Examples include ruthenium on activated carbon, but Raney nickel is considered the most promising candidate for commercialization.
The conversion of a carbohydrate to alkylene glycol involves complex reactions. It has been shown in M. Zheng et al., Chin. J. Catal., 35 (2014) 602-613, that lower concentrations of carbohydrate and high reaction temperatures, i.e. above 200° C., are beneficial to ethylene glycol production. This appears to be confirmed in WO 2014/161852, containing experiments wherein glucose solutions with increasing glucose concentrations, ranging from 1% wt to 6% wt, were contacted with hydrogen in the presence of a catalyst system comprising tungsten and ruthenium. The higher the glucose concentration was, the lower the yield of ethylene glycol became. In order to remedy this disadvantageous effect, it is proposed in WO 2014/161852 to contact a first small portion of the carbohydrate with hydrogen and the catalyst in a solution with a carbohydrate concentration of less than 2% wt, and only when the first portion has reacted, to add further portions of the carbohydrate. In this respect the process is similar to the semi-continuous reactions described in G. Zhao et al., Ind. Eng. Chem. Res., 2013, 52, 9566-9572. Both WO 2014/161852 and G. Zhao et al. in Ind. Eng. Chem. Res., 2013, 52, 9566-9572, mention that, in addition to ethylene glycol, 1,2-butane diol (butylene glycol) is produced. The relative amount of butylene glycol can be in the order of 10%, based on the yield of ethylene glycol. Since butylene glycol and ethylene glycol form an azeotrope, it is difficult to separate the compounds easily via distillation.
As indicated above, alkylene glycols find application in a range of products. An important application is its use as monomer in the production of polyesters. Especially when alkylene glycols, such as ethylene glycol, are used in the production of polyesters, the alkylene glycol must be pure. As shown in e.g. WO 2014/161852, the conversion of hexose-containing carbohydrates, such as glucose and cellulose yields a mixture of ethylene glycol, propylene glycol and some butylene glycol.
The yields of ethylene glycol in the process according to WO 2014/161852 are in the order of 22 to about 40%, based on the amount of glucose used as starting material. In addition to other alkylene glycols, such as propylene glycol and butylene glycol, several other by-products are thus formed.